Transmitters are widely used in several application areas, like mobile radio or satellite technology. It is important for systems of such technology areas that energy consumption is small due to the limited power supply. Thus, it is an ongoing concern of the industry to reduce power consumption of transmitters.
One possible solution for reducing power consumption is to use power efficient amplifiers, like switching power amplifiers. Several ways are known in prior art for controlling a switching power amplifier. However, switching power amplifiers has to be driven by a signal comprising a good linearity. Due to this fact suitable modulation methods, like pulse width modulation (PWM), pulse position modulation (PPM) or pulse density modulation (PDM) are essential.
Moreover, it is an ongoing concern of the industry to increase the transfer rate. One possibility is to use the amplitude and the phase as information carriers to achieve a more efficient modulation scheme. However, this implicates linearity issues, in particular in connection with switching power amplifiers.
A switching power amplifier can be driven via its supply voltage, wherein the supply voltage may depend on the amplitude component. Another prior art solution for driving the switching power amplifier is to use a two-level PWM signal, which can be generated by comparing the amplitude modulated carrier signal and the phase modulated carrier signal with a saw tooth signal having a fundamental frequency of at least twice the carrier frequency. A further prior art modulation type may use a two-level PDM signal, which can be generated by a band-pass sigma-delta. All these solutions comprise an analogue circuitry. However, digital modulation may be desirable.
As already mentioned, a suitable solution may to convert a complex signal to a phase modulated signal comprising a constant envelope. Such signals can be produced by pre-distorting four base band signals. However, one issue is that an analogue circuitry is required and matching requirements may be high. Furthermore, a digital realization may comprise the drawback of extremely high sampling frequencies for achieving a desired resolution.
It is one object of the present application to provide a digital modulator to avoid analogue circuitry. It is a further object to provide a digital modulator with reduced required sampling frequencies. A further object is to avoid a significant loss in the resolution of the digital modulator. It is another object of the present application to provide reduced power consumption of the digital modulator.